Accompanying recent advances in computerization has been an increasing demand for personal digital assistants having processing capabilities comparable to that of earlier personal computers. As a result, high-resolution and high-quality video display devices fulfilling the requirements of reduced thickness and weight, a wider viewing angle, and smaller power consumption are desired.
In order to meet desired requirements, display devices (or displays) in which thin film active elements (thin film transistors, or simply referred to as TFTs) are formed in a matrix on a glass substrate, and electrooptic elements formed thereon are controlled by the TFT to be driven are being actively developed.
The substrate for forming the thin film active elements is usually formed by providing and patterning a semiconductor film of, for example, amorphous silicon or polysilicon, and making connections using metal wiring lines. Due to differences in electric properties between the thin film active elements, a driving integrated circuit (IC) must be separately provided when amorphous silicon is used while a driving circuit can be formed on the substrate when polysilicon is used.
In liquid crystal displays (hereafter simply referred to as LCDs) currently in widespread use, amorphous silicon is most commonly used in larger size displays, while polysilicon has become the standard material used for medium and small sizes because it is suited for providing higher resolution.
Regarding organic electroluminescence (organic EL) type displays having characteristics of self-emissiveness, reduced thickness and weight, and a wider viewing angle, only polysilicon type displays are mass produced.
Generally, an organic EL element is combined with a TFT to utilize a voltage current controlling function thereof for controlling a current. The voltage current controlling function is the function to control a current between a source and a drain by applying a voltage to a gate terminal of a TFT. By setting a voltage of the gate terminal of the TFT supplying a driving current to the organic EL element to a voltage in accordance with luminance data (tone data), the driving current in accordance with luminance data can be supplied to the organic EL element to adjust intensity of emitted light, and to therefore display an image in a desired tone.
However, when such a configuration is employed, the intensity of light emitted from the organic EL element is very sensitive to the properties of the TFTs. Polysilicon TFTs, particularly so-called “low-temperature polysilicon TFTs”, i.e. TFTs formed through a low temperature process, tend to exhibit relatively significant variation in electric properties even between adjoining pixels, which is one of the main factors for deterioration in display quality, particularly in display uniformity on the screen, of the organic EL displays.
One conventional technique for addressing this problem is disclosed in Patent Document 1 (Japanese Patent Laid-Open Publication No. 2002-297094). According to this conventional technique, a polysilicon TFT for driving the organic EL element is used as a switch. Variation in properties is suppressed by operating the switch only in two modes of ON and OFF (digital driving), and multiple tones can be achieved by controlling the length of the ON period.
However, when such a polysilicon TFT is used as a switch to drive the organic EL element based on whether or not to apply a voltage thereto, a driving voltage is raised in relatively a short time, and an ON current is decreased due to deterioration of the organic EL element with time. The pixel with the reduced ON current is visible on the display due to burn-in, and the useable lifetime of the display device shortened.